Bimetallic protective coating for iron tubes



April 28, 1931. T. BENNETT 1,802,695

BIMETALLIC PROTECTVE COATING FOR IRON TUBES Filed Jan. 21, 1925' Patented Apr. 28, 1931 UNITTED sTAr-:s

PATENT OFFICE IRVIN G T. BENNETT, OF BROOKLYN, NEW YORK, ASSIGNOR, BY MESNE ASSIGNMENTS, TO METROPOLITAN ENGINEEBIG COMPANY, A CORPORATION OF NEW YORK BIMETALLIC PBOTECIVE COATING FOR IRON TUBES Original application filed .Tune 26, 1924, Serial Io. 722,462. Divided and this application filed January 21, IME. Serial No. 3,720.

. tures. The present application is a division thereof, covering the product.

When steel and other metals are subjected m to high temperatures they tend to oxidize more rapidly than at ordinary atmospheric temperatures. Various methods have been devised for treating steel to increase its durabilitv under such conditions The most successful'of such methods have involved the formation upon the surface of the steel of a continuous, adherent film of oxide, which is nonporous and of low oxygen solubility. This film thus protects the underlying metal from further oxidation.

In practice I have found that the best of .the known-methods is effective only at moderately high temperatures, and that at exceedingly high temperatures the surface protection is not durable. My present invention aims to provide a surface which shall be durable at exceedingly high temperatures and which can be economically applied to steel,

or iron, or similar metals; such as, for example, low carbon steel. I have demonstrated its success particularly with boiler tubes made of steel containing` 0.15 to 0.20 percent carbon. `The invention is of particular value in connection with tubes provided with fins, as described in application No. 715,369, and other applications, of Thomas E. Murray These tubes have longitudinal fins welded on to them and are in some cases located in the boiler furnacc and exposed to the radiant heat of the burning fuel, so that they may be heated to an exceedingly high temperature, approaching 2500 degrees F; By treating such fins in accordance with the present invention, their durability under such conditions is greatly increased. v

The articles of steel protected bythe process are adapted also to various other uses, having a surface -which is practically non- `corrodible under ordinary conditions. The treatment may, in fact, be so applied as to make the articles practically insoluble in all acids, except for very slow solubility in aqua regia and hydrochloric acid. The accompanying drawings illustrate an embodiment of the invention.

F ig. l is a cross-section of a round rod ilbon, is electroplated to form a coating of chromium 2. The-rod thus produced is then electroplated to provide a second coating 3 of nickel.

The article, with these coatings deposited thereon, is then heated for two hours or more to a temperature in the range 1800 degrees F. to 2500 degrees F. in an oxidizing atmosphere. In this heating operation there are formed apparently alloys as indicated n F lg. 2. The outer surface is covered by a very thin continuous film of extremely adherent oxide indicated by the heavy line 3 which has practically the same coefiicient of expansion as steel, and which is very dense or non-porous. This is probably a mixture of nickel oxide and chromium oxide intimately dispersed and of low oxygen solubility Immediately below this is the coating 4, in all probability an alloy composed chiefly of nickel and chromium, in solid solution, which adheres closely to an inner coating 5 probably composed chiefly of iron and chromium in solid solution, 'which in turn adheres closely to the steel 1.

After heating it will be noted that there are two different layers. But these are not of the same composition as before heating. The outer layer 4 has a high resistance to acids and to extensive oxidation; whenc, and from its appearance, I conclude that it is an alloy of nickel and chromium in solid solution sim ilar to known alloys of these two metals.

The inner coating 5 merges into the steel in an ill-defined line. In some places the microscope indicates the formation of pearlite between this inner coating and the steel foundation. The coating is apparently an -alloy composed chiefly of iron and chromium in solid solution, being hard to etch and resistant to corrosion. Chromium in this inner coating is also indicated by the formation of pearlite referred to, it being known that chromium lowers the eutectoid composition and permits the formation of more pearlite than in the case of ordinary carbon steels of a similar carbon content.

Chromiu n and iron alloy in the solid state when heated together under pro er condimium iron alloy, which directly protects the steel. It 'is probable that migration of the chromium and nickel into each other is more rapid than the migration of the chromium and iron into each other. Thus the chromium coating is promptly protected and time is given for the formation of the iron chromium v alloy.

The steel with the chromium and the nickel coating deposited thereon may be considered the finished product, ready to be put into use where it will be subjected to high temperature. In that case the conditions of use will fix'the coatings by the oxidizing and alloyin effects which have been referred to.

I r the article coated with chromium and nickel maybe heated to fix the coatings as described before it is put into use. In heat- 'ing the coated articles it ispreferable to use an oxidizing atmosphere for, as previously mentioned, the success of the process depends 4 on the formation of a proper oxide coating together with alloyingeflects produced by' the heating.

Chromium is preferred because it is highly resistant to hea't and because it combines re adily with iron to form highly non-Corrosive alloys,some commonlyknown asstainless steel.

Nickel is preferred for .the outer coating because it readily combines with chromium to form' highly resistant alloys However, other resistant metals or metals that alloy with chromium to frm resistant alloys may be employed. The invention may be applied,`

however, with other materials than chromium and nickel. Also, though I have found electroplating a convenient method of applying the metals, yet they may be apphed in ather known or su table ways.

down and expose 'the underlying steel.

The process is not confined to a single coating of each metal. There may be'several coats of each metal, the two being deposited alternately; and indeed other metals may be added.

As an example of the method of depositing the chromium, a bath used may be of the following composition:

Chromic acid 200-250 grms chromium sulphate 4 grams Water 1000 cc.

Anode of chromium Cathode of steel e Cathode current denslty per sq. ft 100 amps. Solution E. M. F 2-3 volts Ihave found that two hours in such a bath will give a satisfactory deposit, and that agitation will aid this deposition.

An example of the method of depositing the nickel involves the use of the following bath:

Two hours in this bath gives a satisfactory deposit.

In using a greater'number of coatings I propose, after the first compound coating of chromium and nickel, to coat in succession with chromium, iron, chromium and nickel, repeating this series as desired; the purpose of the iron being to alloy with the chromium on either side of it so that the heated article will consist of several laye's of the two different alloys. Though the above order of deposition of the various metals is preferred, any other order which gives satisfactory results may be used.

I have subjected test pieces made as above with a single coating of the chromium and nickel to temperatures of 1800 to 2200 degrees F. for two hours in an oxidizing atmosphere only about one ten-thousandtl of an inch;

whereas the same steel, untreated, subjected to the same test was oxidized'to a depth of from three hundredths to Seven hundredths of an inch and badly scaled. With a tem-' perature of about 2500 degrees F. for half an hour ordinary steel was badly scaled. The same steel treated as above showed no oxidation, and in the case of steel treated by a well known commercial process the aluminum oxid coating was startingto break I have subjected test pieces made a's above with a single coating 'of the chromium and nickeh to temperatures of 1800 to 2300 de 'ees F. for 57 6 hours in an' oXidizing atmosp ere and found the article to maintain a' good looking surface with oxidation to a depth of only.

one ten-thousandth of an inch; whereas, the

4 boiler tube.7 the protective coating being indicated on the exposed surface 8 of the fins. Another advantage in this connection is that the coating 'does not substantially interfere with the heat-conducting property of the article.

I have subjected test pieces also to acids to determine their resistance to corrosion by acid vapors or liquids, and have found the corrosion-resisting and acid-resisting properties of the coated steel to be very high. Taking a sample which had been electroplated as above described and then heated in an oxidizing atmosphere to cause the oxidizing and alloying action, I immersed it in various acids and Compared the results with those obtained by similar tests on commercial acidresisting metals, chromium, nickel and alloys of nickel and chromium and of chromium and iron. Steel treated in a'ccordance with the invention and heated to 1832 degrees F. for two hours showed aresistance to corrosion equal to that of chromium in all acids except concentrated hydrochloric acid in which it far surpassed chromium, being practically insoluble while the chromium rapidly disolved. In fact, the protective coating on the steel treated according to this invention may be made to approach platinum in resisting acids. This resistance is due, in all probability, chiefly to insolublity of the oxide film and, secondly, to the two alloys underlying this film which are in themselves very similar to commercial resistant alloys.

The corrosion resisting properties of the treated steel make it valuable in laboratories and workshop where strong acids are. employed or where, because of the corrosive fumes present, it would be-impossible to use untreated steel.

-The invention may be applied also to the protection of a piece of metal made up for practieally -or substantially its entire thickness. by this process; that is, starting with a -small core and building up with successive coatings to the desired size. For example, the

fins 8 of F ig. '3 might be made up through.-

out all or -the greater part of their thickness of' the successive coatings describedabove.

I have stated above my theory as to what takes place in the application of my process.

'And it seems clear that the process is dependent on the diflusion' 'of metals in the solid series of alloys which are extremel resistant state, resulting in the formation o alloys or to oxidation and to acids. But such diflusion s dependent on the temperature applied and the length of time of its application, and the product may vary in this respect within a coating of nic rect results of the treatment to be different i for each of these base metals. The invention contemplates not only the use of nickel but also the use in the same way' of equivalent metals; that is, metals closely allied in their Chemical and physical properties, belonging to the same Chemical group and occupying similar places in Mendelejefi s periodic table.

Though I have described with great particularity of detail certain embodiments of my invention, it is not to be understood therefrom that the invention is restricted to the embodiments illustrated. Various modificathe art without departing from the invantion as defined in the following claims.

What I claim is 1. The herein described product comprising ferrous metal with a coating of chromium and with outer coating of a metal of the nickel group over the chromium.

2. The herein described product comprising ferrous metal with successive protective layers, first one of chromium and an outer one of nickel on the chromium layer.

3. The herein described product comprising ferrous metal with 'a coating of an ironchromium alloy and an outer coating of an alloy of chromium and a metal of the nickel group. 4

4. The herein'described product compris- ,ing ferrous metal with a coating of chromium and with an outer coating of a metal of the nickel group over the chromium, and an outermost coating of an adherent oxide;

.tions thereof may be made by those skilled in 5. A tube provided with a fin of ferrous metal having a coating of chromium and 'an outer coating of a, metal of the nickel group over the chromium and an outermost coating x of an adherent oxide. v, u

7. A water tube for boilers provided with a-fin to be exposed to the fire, said fin being of `ferrous metal and having' a 'surface alloyed with a coatingof chromium and an out'ermost elallo ed with said chromium. In witness whereo I have hereunto signed my name.

.IRVING T. BENNETT. 

